Experiments were carried out to determine the effect of adenovirus on SV40 replication, transcription and translation during coinfection of primary African green monkey kidney cells. Coinfection of these cells with SV40, concomitantly or prior to adenovirus infection, results in the abolishment of the block to adenovirus growth in monkey cells. Some evidence has pointed to the posttranscriptional block as a result of inefficient processing (splicing) of late adenovirus mRNA. The A gene product of SV40, T-antigen, has been shown to be responsible for this adenovirus "helper function." Although adenovirus can inhibit SV40 DNA replication, there is no effect on SV40 transcription and translation at a low adenovirus multiplicity of infection. Further studies indicated that SV40 could complement a conditional lethal DNA-negative adenovirus mutant for replication, and adenovirus could modulate the overproduction of early mRNA from an SV40 conditional lethal DNA-negative mutant at the restrictive temperature.